This invention relates to RF load and source pull testing of medium and high power RF transistors and amplifiers using remotely controlled electro-mechanical impedance tuners.
Modern design of high power and low noise RF amplifiers and mixers, used in various communication systems, requires accurate knowledge of the active device's (microwave transistor's) characteristics. In such circuits, it is insufficient for the transistors, which operate in their highly non-linear regime, close to power saturation, to be described using non-linear numeric models.
A popular method for testing and characterizing such microwave components (transistors) is “load pull” or “source pull”. Load/Source pull is a measurement technique employing microwave tuners and other microwave test equipment (FIG. 1), such as signal source (1), input and output tuner (2, 4), power meter (5) and test fixture (3) which comprises the device under test (DUT). The tuners and equipment are controlled by a computer (6) via digital cables (7, 8, 9). The microwave impedance tuners are used in order to manipulate the microwave impedance conditions under which the DUT (transistor) is tested (see ref. 1); this document refers hence to “impedance tuners”, in order to make a clear distinction to “tuned receivers (radios)”, popularly called elsewhere also “tuners” because of the included tuning circuits (see ref. 2).
Electro-mechanical impedance tuners (FIG. 2) in the frequency range between 100 MHz and 60 GHz use the slide-screw concept and include a slabline (24) with a center conductor (23) and one or more mobile carriages (28) which carry a motor (20), a vertical axis (21) and control the vertical position (216) of a reflective probe (22). The carriages are moved horizontally (217) by additional motors and gear (27). The signal enters into one port (25), the “test port”, and exits from the other port (26), the “idle port”; in the case of the input tuner (2) the test port is the signal exit port, whereas in the case of the output tuner the test port is the signal entry port. The entire tuner mechanism is, typically, integrated in a solid housing (215) since mechanical precision is of highest priority.
The typical configuration of the core of the tuner is shown in FIGS. 3 and 4; it comprises, in general, a slotted transmission airline (31, 44) and a number of metallic parallel tuning elements (30, 41) also called “tuning” probes, “reflective” probes or “slugs”, which are coupled with the center conductor to an adjustable degree, depending from very low coupling (when the probe is withdrawn) to very strong coupling (when the probe is within Corona discharge distance from the center conductor, see ref. 4). “Sampling” probes on the other hand are loosely coupled with the center conductor and only detect a small amount of the signal power.
When the tuning probes approach (34, 44) the center conductor (32, 43) of the slabline (31, 44) and moved along the axis (45) of the slabline, they modify the amplitude and phase of the reflection factors, covering parts or the totality of the Smith chart (the normalized reflection factor area). The relation between reflection factor and impedance is given by GAMMA=(Z−Zo)/(Z+Zo), where Z is the complex impedance Z=R+jX and Zo is the characteristic impedance. A typical value used for Zo is 50 Ohms (see ref. 3).
Up to now such tuning metallic probes (slugs) have been made in a cubical form (30, 41) with a concave bottom (35) which allows capturing, when approaching the center conductor (32, 43), the electric field which is concentrated in the area (36) between the center conductor (32) and the ground planes of the slabline (31) (FIGS. 3 and 13), where the center conductor is closest to the internal surface of sidewalls (37). This field capturing allows creating high and controllable reflection factors. Contact of the probes with the sidewalls (37) is critical. It can be either capacitive or galvanic. If the contact is capacitive, the surface of the probes and/or the sidewalls of the slabline must be electrically insulated. This can be done using chemical process such as “anodization” (see ref. 8). Nevertheless capacitive contact means extreme requirement in sidewall planarity and straightness to keep the quasi sliding contact constant for the whole length and depth of the slabline as the probe travels.
Galvanic contact is safer, but requires a spring mechanism to allow for constant pressure of the probe on the sidewalls. The two possible scenarios (capacitive and galvanic contact) are shown in FIG. 5; FIG. 5a) shows a probe (56) with galvanic ground contact (50) and FIG. 5b) a probe (55) with capacitive contact (54) with the slabline walls (56, 57). Probe 5a) must have a springing mechanism (51) which is created by machining a horizontal hole and slot (52) into the body of the probe. Probe 5b) can be massive (55).